Populations ingesting a high fiber content in their diet are reputed to have a lower incidence of cholesterol cholelithiasis. In order to investigate this, 5 subjects had biliary lipid parameters measured both before and then right after an ingestion of a diet containing large quantities of bran. Gallbladder bile was sampled in a routine manner and lipid composition as well as bile acid composition were determined on samples both before and after bran ingestion. The ingestion of a high bran diet did not induce any changes relative to cholesterol composition of gallbladder bile or in any parameters of bile acid composition. It seems unlikely, therefore, that the lowered incidence of cholesterol cholelithiasis in populations eating a large amount of fiber are mediated through changes in biliary cholesterol content.